1. New Zealand (NZ) desperately wants closer economic ties to the US but is constrained by public sentiment
Because New Zealand is primarily a low value added commodity export based economy the NZ Government and NZ businesses highly covet lower trade barriers. However NZ's anti-nuke stance and lack of military spend means that NZ will never get free trade agreement. Therefore anything else the NZ Government can do to appease US counterparts wins NZ small favors. This is why NZ is very much in bed with US spooks and basically has a US intelligence facility in Waihopai.
I firmly believe that this was a primary reason for the NZ Government's willingness to let their US Government counterparts to run the operation on Kim Dotcom.
2. NZ has been trying to woo the US film and TV industry
The NZ Government has been doing its best to entice the US film and TV industry even before the success of The Lord of the Rings (circa 1995 with Hercules: The Legendary Journeys).
I'm of the opinion that the heavy handed nature of the raid on Kim Dotcom was to send a message the NZ Government takes the rights of film and TV studios seriously.
That said, I think the NZ Government didn't properly gauge how far public opinion would swing against them for being so heavy handed against Kim Dotcom. Kiwis love an underdog and when the local TV media showed video footage of the raid on Kim Dotcom complete with helicopters and SUVs it certainly looked like an absurd Hollywood action movie. Also the use of the Government Communications Security Bureau (GCSB) to spy on Kim Dotcom unnerved many Kiwi's sense of privacy.
After this recent ruling I'm not sure how this whole saga will play out. I imagine that with with his personal freedom at stake, Kim Dotcom will keep pressing his case through to the Supreme Court of New Zealand.